This invention relates to liquid cooling systems for internal combustion engines and more particularly to pressurized systems equipped with relief valves for venting the system if predetermined maximum operating pressures are exceeded.
It has long been known to pressurize or increase the maximum operating pressure of a given cooling system as a means of getting an increase in cooling capacity without increasing physical size of the system. An increase of pressure elevates the coolant boiling point in accordance with the well known laws of physics so that higher operating temperatures are possible without undesirable boiling of the coolant or related problems such as circulating pump cavitation and overflow and loss of coolant. With higher temperature differentials between coolant and ambient air at the radiator core, cooling capacity of the system is increased.
In a typical pressurized system, however, only a single relief pressure is provided and this pressure, of course, must be relatively high, consistent with the maximum cooling capacity needed for the most severe operating conditions of the particular engine installation. Further, it is characteristic of such systems that they operate at or near this relatively high relief pressure during most of their operating lives and thus, much of the time, away from an optimum combination of coolant pressure and temperature. The life of cooling system components such as the radiator core, radiator hoses and water pump seals are shortened, comparatively, when subjected frequently to operating cycles with unnecessarily high coolant temperatures and pressures.
Further, although nominally increasing the overall cooling capacity of a given system, increasing its maximum operating or relief pressure may actually have an adverse effect on cooling at certain critical points in the engine, particularly in systems where a significant amount of phase-change cooling occurs. For example, the most efficient cooling occurs at an engine cylinder wall when conditions are such that some phase transformation takes place--that is, when heat from the cylinder wall is sufficient to raise the temperature of the coolant in contact with it to its incipient or nucleate boiling point. An increase in the operating pressure of a given system elevates the coolant boiling point, and the coolant temperature rise at the cylinder wall may then be sufficient to produce these optimum heat transfer conditions only in rarely met extreme operating conditions and, in fact, during normal operation there may be an actual decrease in heat transfer from the cylinder wall to the coolant. The resulting increases in the cylinder wall and piston temperatures and in cylinder peak firing pressures may, for example, lead to early fatigue failures in pistons which are typically made of material which has lower fatigue strength at elevated temperatures. In addition, lubricating oil temperatures are higher and there is an increased rate of oil contamination.
It has also been known to provide cooling systems in which system pressure varies with engine operating conditions in the normal working range, maximum operating pressure being limited by a conventional pressure-cap relief valve. For example, U.S. Pat. No. 3,765,383, Birdwell, discloses a closed cooling system, completely filled with coolant, of a type sometimes called a recovery system. A bellows-like accumulator is provided to accommodate the expansion or "overflow" of coolant from the radiator which may occur as the engine warms up. The expandable accumulator is mechanically restrained in such a way that the rate of increase of system pressure is at first slow but eventually is caused to rise more rapidly as maximum permissible coolant temperatures are approached. Clearly such a variable pressure system has a greater potential for providing heat transfer conditions at critical points closer to optimum over a wider range of operating conditions than a conventional system having only a single maximum operating or relief pressure. However, this is a passive system in which pressure, as a function of temperature, is a dependent variable. The system is without feedback or self-correcting ability, and is dependent upon such factors as careful maintenance of coolant fill level and coolant composition for repeatability of a predetermined pressure/temperature characteristic.